Stabilizer for automatic volume control circuits



Sept. 4, 1945. J. o. PARR, JR

STABILI ZER FOR AUTOMATIC VOLUME Filed Aug. 4, 1943 CONTROL CIRCUITS 2 Sheets-Sheet l METER ALLA m E- u y g g 1 I 0 g i u A lll i|||| ||||||||il||lllmlillllll $5 I I g I 3mm r r Josephus O- pen.

1945- J. o. FARR, JR

STABILIZER FOR AUTOMATIC VOLUME CONTROL CIRCUITS Filed Aug. 4, 1943 2 Sheets-Sheet 2 :k: k w Q x Q g s 1 Q Q Jobepkua Parr, Jr.

Patented Sept. 4, 1945 UNITED STATES PATENT OFFICE BTABILIZIR FOB AUTOMATIC VOLUME CONTROL CIRCUITS Josephus o. Parr, Jr., San Antonio, Tex assignor to Olive 8. Petty, San Antonio, Tex.

Application August 4, 1943, Serial No. 497,371

9 Claims.

This invention relates to methods of and apparatus for conducting seismic surveys for use in connection with geophysical prospecting, and more particularly to apparatus and methods for amplifying and recording the seismic wave trains s as they arrive at one or more reception points.

In the amplification and recording of seismic energy, it is customary practice to provide means for automatically regulating the sensitivity of the amplifying and recording equipment in order that 10 waves of widely varying amplitude may all be received and recorded by sensitive apparatus on a record sheet or other medium of convenient dimensions. I have described one such an arrange-- ment in my prior application for United States Letters Patent Serial No. 411,479, filed September 19; 1941, the circuit shown therein being basically similar to that illustrated and described in the patent issued jointly to Olive S. Petty and me,

No. 2,318,624,, granted May 11, 1943. 'Iheprior application and patent both disclose, in association with a seismometer or other device for converting seismic impulses into electrical wave form signal energy, an amplifier for such signal energy, the degree of amplification effected in the ampliiler being automatically regulated in response to change in amplitude of the received seismic waves.

Each of these disclosures further includes a showing of means whereby the regulation of amplii'ier sensitivity following a given change in amplitude of received energy is somewhat retarded, so as to reduce the distortion of the record which would otherwise result. While the introduction of some time delay circuit for this purpose is altogether desirable, it is usually preferable to retard the application of sensitivity control no more than is absolutely necessary to prevent serious distortion of the record, since it is obvious that prolonged delay in the reduction of sensitivity following the'arrival of signals of excessive amplitude inevitably permits the signal energy to build up to amplitudes which cannotbe recorded on a record sheet of limited width, and a portion of the record is therefore sacrificed before the amplifier sensitivity can be reduced.

It is accordingly an object of the present invention to provide a method and'means whereby the application of sensitivity control, or automatic volume control, is effected with a minimum of delay following an increase in amplitude of the incombing signal energy of suiiicient magnitude to require such compensation.

More specifically, it is an object of the invention to provide, in an amplifier of the type in which automatic gain control is effected by the application to an amplifier valve grid of a potential derived from the signal energy. means for minimizing the distortion normally resulting from prompt application of the controlling poten- I tial to the grid.

The basis of the present invention is the discovery thatthe principal source of distortion in amplifiers of the type described is the result of feeding to the control grid of the amplifier valve a potential or bias on which is superimposed a pulsating voltage, sometimes referred to as a ripple component, caused by deriving the bias A from the rapidly oscillating signal energy. In my aforesaid application and patent, the time delay circuit hereinbefore referred to serves as a filter to reduce substantially the effect of the ripple component. However, any reduction in the size of the condensers employed in the filter tends to increase the effect of the ripple component, and since this ripple component is then amplified by one or more stages of amplification following the controlled valve, and is again returned to the AVC circuit to further increase the control potential, the effect of the ripple component is gradually increased during the reception of signal energy of large amplitude, until a condition of oscillation is approached, with the result that the distortion is excessively large.

I have found that the ripple component may be minimized and substantially eliminated by the use of an additional circuit, supplied with energy from the amplifier, and so constructed and arranged as to combine with the current which supplies the control bias a pulsating current of comparable magnitude and opposite phase, whereby the ripple component is substantially cancelled. For example, a portion of the energy in the A. V. C. circuit may be applied to a transformer which passes the superimposed pulsating current, the output of the transformer being so returned to the A. V. C. circuit as to provide a voltage which is combined with the ripple component in phase drawings in which:

Figure 1 is a wiring diagram of a system for the reception of seismic waves, illustrating one form of amplifier to which the invention may be applied; and

Figures 2 and 3 illustrate modified forms of a part of the circuit shown in Figure 1.

For the purpose of illustration I have shown one method of applying the invention to a circuit essentially similar to that represented in my prior application and patent hereinbefore mentioned. It will be appreciated, however, that the invention is by no means limited to this type of circuit and that the underlying principle whereby th ripple component of a sensitivity control voltage is minimized or cancelled out by the combination therewith of a comparable pulsating voltage, out of phase with the original ripple component, may be utilized in conjunction with the widely varying types of seismic apparatus.

Referring now to Figure 1 of the drawings, there is shown a detector or seismometer ll of any suitable type adapted to convert seismic energy to wave form electrical signals. These signals are fed into a suitable output circuit which includes the primary II of a transformer.

the secondary 21 of which is shunted by a pair of variable resistors 28 and II, one for coarse and one for fine adjustment, for manually setting the level of the signal applied to the amplifier.

The secondary 21 feeds into the cathode and rid of the thermionic valve II which constitutes a simple triode amplifier having the anode 3|, grid 31, and heated cathode I! which it shares in common with the elements of the second stage amplifier valve indicated at 34. A suitable biasing battery 3! is arranged in the grid circuit of the triod amplifier. This amplifier is coupled to the second stage amplifier by means of an anode coupling resistor 38 and condenser 81. The anode It as well as the anode II of the second stage valve receive their voltage supply through the common conductor ll connected to the positive end of the battery ll. A tap it in this battery supplies the screen grid 42 of the second stage amplifier with a somewhat lower potential, permitting more gain and improved automatic volume control. The control grid 43' of the second stage amplifier is coupled by the condenser 81 tothe anode of the taken in conjunction with the accompanying ment of a galvanometer associated with and forming part of a recorder ll. Another secondary It takes off a portion of the output of the amplifier for supply to the automatic volume control valve as will be further described hereinafter. The condenser 08 between the anode ll of the third stage valve and ground serves to by-pass high frequencies and to resonate the output transformer to assist in the desired filter action. At the same time, this condenser performs the important function of assisting in the damping of the moving element of the galvanometer.

In the same envelope with valve 0 is a triode, which may for convenience be referred to as the A. V. C. valve, comprising anode 1, grid II, and- {the common heated cathode I! which functions also with the elements of the third stage amplifier. The anode I1 is connected by wire II to a separate source II of anode voltage, the negative pole or which leads through variable resistors I02, m and I, and to ground through contller'lsers Ii and I3 and through resistors Ill and The grid ll ofthe A. V. C. valve is fed from the secondary 82 of the output transformer II, the opposite end of which is connected by conductor TI to the slider of a potentiometer ll energized from the portion of battery ll between negative terminal II and adjustable tap ll, through the contacts ll of a relay, the winding 82 of which is in series with the heaters of valves 30 and 40.

The slider on the potentiometer I0 is aet'to supply a sumciently negative bias to the control grid I. of the A. V. C. valve to normally prevent the flow of any anode current in that valve. Preferably the valve is thus biased substantially beyond cut-off, and flow of anode current occurs only during positive swings of the output signal of sufficient magnitude to drive the grid across the cut-off point toward the positive side. Thus in the anode circuit of the A. V. C. valve, the

first stage and receives its bias through resistor from a tap on battery 40. An adjustable coupling condenser 41 and mode resistor ll provide the coupling between the second stage anode a and the control grid 4: of the third stage amplifier valve 4!. Condenser l'l acts in the nature of a filter, first to avoid fiow of anode current through the primary winding '52 of the interstage coupling transformer 88, thus increasing the transformer life and permitting its design to be more efiicient, and second, to vary the frequency response of the amplifier, giving it the highest gain on the desired frequencies and very low gain on undesired frequencies, which effect comes from resonating the transformer primary. J

4 The secondary winding ll of the interstage transformer 83 connects between the control grid ll and the bias battery ll. The outputfrom the third stage is fed through the primary to of transformer 58, one ndary ll of which delivers the amplified waves to the moving elepositive pole of battery 48 is substantially at ground potential. The bias on grids 0 and ll of the second and third stage amplifier valves is therefore determined by the potential of the battery ll and the gain of the amplifier is fixed thereby. However, when current fiows in the anode circuit of the A. V. C. valve through the resistors III, III and I, and through resistors ill and Ill, the potential or the conductor ll becomes negative with respect to ground. and the bias on the grids ll and II is thereby rendered more negative to reduce the gain of the amplifier.

It has already been explained how the reception of waves of greater amplitude than a predetermined value causes functioning of the A. V. C. valve. If this value is set at approximately the amplitude of the first received refiected waves, then the operation of the A. V. C. will cease when the refiected waves begin to arrive at the seismometer and if not means are taken to prevent it, the gain of the amplifier will become normal forthe remainder of the operation. This is not desirable since the setting for the original gain would have to be sufficiently low to maintain the-first portion of the reflected waves at a reasonable size and the later portion would then be too small for use. The-normal gain is therefore set to produce a greater than necessary output from the first of the reflected waves to arrive. Thegain is prevented, however, from returning to normal after being reduced by the large primary-waves until close to the end of the whole record.

Thus on the arrival of the earlier waves of excessively large amplitude, the grid of the 1 C. valve is driven suillciently positive by the ikal energy derived from the output transforme secondary 02 to cause a flow of current to the anode 01, thereby charging the condensers BI and 03 and applying a more negative bias to the grids 00 and 00 of the amplifier valves to reduce the gain of the amplifier. ting of the resistor determining the bias on the A. V. C.- valve should be such that each of the swings of the signals derived from these excessively large waves causes the A. V. C. valve to function and therefore the charging of condensers II and 80 continues until such waves have passed. I

As hereinbefore indicated, the A. V. C. system just described, it operated in such a manner as to produce no noticeable distortion in the record,

I is sufilciently delayed in its action to prevent The setdenser 0| (mfd.) Xr'esistor I02 (ohms) xturns ratio of windings ill to I01.

As hereinbefore indicated, resistor I00 is not critical and may be omitted, but is preferably,

. The operation of the circuit just described will be apparent since the load resistors ill and I0! are large in comparison to the impedance of the condensers I00 and H, and the current in both circuits will be in phase with the applied voltage. Consequently the ripple voltages appearing across these condensers are in phase with the voltage applied to the network between terminal rapid reduction of large voltage swings. Should the action be too rapid, the ripple component is fed to the grids 40 and 00 of the amplifier valves and, after further amplification, is returned to the grid 00 of the A. V. 'C. valve andv thence to the condensers 0i and 93, with sub-v stantially increased amplitude. As pointed out hereinbefore, this building up of the ripple component is a major factor in the distortion of the record, especially when it is attempted to reduce the time required to aifect the sensitivity of the amplifier following the arrival of signals of high amplitude. be extremely effective in reducing this ripple component to a minimum is shownin Figure l of the drawings, involving the introduction in the conduction path for the control signal. of the primary winding I00 of a transformer I00.

When the current of anode 01 increases'or decreases a ripple voltage is fed through resistance IM and will charge condenser I00. This voltage across winding I01 of transformer I00 will induce a voltage across winding I00. By proper selection of the constants, the ripple volta e appearing across winding I00 willbe equal in magnitude and 180' out of phase with the ripple voltage appearing across condenser 91. Hence practically no ripple voltage will be fed through resistances I00 and I00 to condenser 00.

For all seismic-frequencies, the impedance of resistor |0l should be quite large in comparison to the impedance of winding I01, which in turn should be quite large in comparison to resistor I00 and to the impedance of condenser I00. This condition is readily obtainable in seismic amplifiers since the resistance of "I can be'in the neighborhood of 500,000 ohms, the reactance of I01 at the lowest seismic frequencies maybe 50,000 ohms, the resistance of'l00 can be 5,000 ohms, and the reactance of I05 may be 500 ohms,

these figures being given merely by way of example but representing effective and wholly practical conditions of operation. A further desirable condition is that the impedance of re- One method which I have found to iii and the ground, and since this is true regardless of the frequency of the applied voltage, it is unnecessary to compensate for variation in frequency.

It may be noted that it is desirable that the impedance ofwinding I01 be large in comparison to the impedance of condenser Ill, so that no appreciable. phase angle shift will result from the introduction of the winding at this point. Resistors i0! and I00 may be'employed to compensate for any slight departure from the conditions hereinbefore stated, and I00 if preferably variable so that the time constant may be adiusted.

It may be noted that the use of sucha circuit in the A. V. C. filter will decrease the energy fed through the A. V. C. filter network at seismic frequencies (at which frequencies the main amplifier has maximum gain).

In the form of the invention shown in Figure 2 the major portion of the amplifier circuit has been omitted for simplification, only the essential part of the A. V. C. circuit being illustrated, and similar reference characters are employed to designate similarly functioning elements. In this embodiment of the invention, the negative terminal ill of battery. is grounded through resistor "I and is connected through variable resistor 202 to one winding 20! of a transformer 2001 The winding 20! is connected to the winding 203 as shown, and this connection is applied to one plate of condenser 200, the other plate being grounded. The opposite end of winding 200 is connected through variable resistance '2" sistor I02 be quite large in comparison to the 1 impedance of condenser 0|. The condition for complete cancellation of the ripple component may be expressed by the following equation: Condenser I00 (mfd.) xresistor l0l (ohms) =conto the positive terminal of battery 00 and thence to the control grid of the amplifier valves as in the embodiment shown in Figure 1. Resistance 201 is also connected through condenser 00 to ground. I

It will be appreciated that by suitable adjust ment of the constants of this circuit the A. C; ripple flowing through winding Ill induces a corresponding ripple voltage in winding 200 which is degrees out of phase with the ripple voltage applied to condenser 200 through winding 20!, so that 7 the pulsating current is substantially cancelled and practically no ripple voltage is applied to the amplifier grids. Variable resistor 201 may be adjusted to control the time constant.

The circuit shown in Figure 2 is highly advantageolis and economical, especially since the best results are secured by the use of a transformer 200 having a relatively high core loss at higher frequencies, andjsuch a transformer is among tion of the amplifier being omitted. In this figure the negative terminal I of battery 1| is grounded through variable resistor 2M and is connected to the cathode of thermionic valve 2|2 through resistance 202, the cathode being connected to ground through condenser 9|. A variable tap on this resistance leads through condenser 2|5 to the grid of valve 2|2, which is suitably biased by resistance 2| The anode 2 IQ of the valve is returned to the cathode through battery 2|8 and resistor 2|9, and the negative terminal of the battery is connected to the positive terminal of battery 46 through resistor 22l.

Since there is normally a plate current flowing in valve 2|2 which produces a voltage drop across resistor 2|9, the negative terminal of battery 2|, is negative with respect to ground and with respect to the valve cathode. When a negative ripple component is applied at the negative terminal of battery 1|, a part of this component is fed through the slider on resistance 202 and through condenser 2|! to the grid of the valve, thus reducing the anode current and the voltage drop in resistor 2|9. The ripple voltage applied to condenser through resistor 202 is thus cancelled out.

It will be appreciated from the foregoing that the present invention improves sensitivity control of seismic amplifiers in a, number of respects, including stabilization =01 the A. V. C. circuit, reduction of distortion due to the operation of the A. V. C. while permitting shortening of the time constant thereof, reduction of the number of valves and accessory equipment required to lessen distortion by other methods, and general simplification'of the circuit while obtaining improved results.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. In apparatus for use in seismic surveying employing a seismometer for convertingdncoming seismic energy into electrical wave form signals, the combination with means for amplifying and recording such signals, said amplifying means including thermionic valve amplifiers, of means for applying to a control grid of at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy so as to' maintain the major portion of the record within usable amplitude limits, said last named means including devices deriving the bias voltage from the signal energy, and means operable by the signal energy for applying to said devices a pulsating voltage substantially in phase opposition with the ripple component of said charge and of comparable.

magnitude, whereby distortion of the record resulting from the feeding to the control grid of charge to said condenser, and means operable by the signal energy for combining with said charge a pulsating voltage in substantial phase opposition with the ripple component of said charge and of comparable magnitude, whereby distortion of the record resulting from the feeding to the control grid of said ripple component is minimized.-

3. In apparatus for use in seismic surveying employing a seismometer for converting incoming seismic energy into electrical wave form signals, the combination with means for amplifying and recording such signals,said amplifying means including thermionic valve amplifiers, of means for applying to a control grid of at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy so as to maintain the major portion of the record within usable amplitude limits, said last named means including a circuit operable in response to increase in signal amplitude above a predetermined value for deriving said bias voltage from the signal energy, and means operable by energy fiowing in said circuit for applying to said circuit 'a pulsating voltage in substantial phase opposition with the ripple component of said bias voltage and of comparable magnitude, whereby distortion of the record resulting from the feeding to the control grid of said ripple component is minimized.

4. In apparatus for use in seismic surveying employing a seismometer for converting incoming seismic energy into electrical wave form signals, the combination with means for amplifying and recording such signals, said amplifying means including thermionic valve amplifiers, of an A. V. C. circuit deriving energy from said signals for applying to a control grid of at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy above a. predetermined minimum value so as to maintain the major portion of the record within usable amplitude limits, and means operable by energy fiow in said A. V. C. circuit for applying to said circuit a pulsating voltage in substantial phase opposition with the ripple component of said bias voltage and of comparable magnitude, whereby distortion of the record resulting from the feeding to the control grid oi said ripple component is minimised,

5. In apparatus for use in seismic surveying employing a seismometer for converting incoming seismic energy into electrical wave form signals, the combination with means for amplifying and recording such signals, said amplifying means including thermionic valve amplifiers, of

an A. V. C. circuit deriving energy from said said ripple component is minimized.

2. In apparatus for use in seismic surveying employing a seismometer for converting incoming seismic energy into electrical wave form signals, the combination with means for amplifying and recording such signals, said amplifying means including thermionic valve amplifiers, of means for applying to a control grid of at least one oi said amplifiers a bias voltage varying autnmstically in response to variation'in amplitude of incoming energy. so as to maintain the major portion or the record within usable amplitude limits, said last named means including a condenser connected to supply the bias voltage, to said grid, and devices deriving from the signal energy a biasing charge and spp fl s such signals for applying to a control grid of at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy above a predetermined minimum value so a to maintain the major portion of the record within usable amplitude limits, and means operable by energy fiow in said A. V. C. circuit for applying to said circuit a pulsating voltage in substantial phase opposition with the ripple component of said bias voltage and of comparable magnitude, whereby distortion of the record resulting from the feeding to the control gridofsaldrippl componentisminimiaedsaid last named means including a translormer having the primary winding thereof included in said circuit, anddevices appl n to said circuit the voltage thereof developed in the secondary winding 6. In apparatus for use in seismic surveying employing a seismometer for converting incoming seismic energy into electrical .wave form signals, the combination with means for amplifying and recording such signals, said amplifying means including thermionic valve amplifiers, of an A. V. C. circuit deriving energy from said signals for ap plying to a control grid of at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy above a predetermined minimum value so as to maintain the major portion of the record within usable amplitude limits, and means operable by energy fiow in said A. V. C. circuit for applying to said circuit'a pulsating voltage in substantial phase opposition with the ripple component of said bias voltage and of comparable magnitude, whereby distortion of the record resulting from the feeding to the control grid of said ripple component is minimized, said last named means including a transformer having the primary and secondary windings thereof included in series in said circuit and so connected that the current flow in one winding is opposed to the current induced therein by the other winding.

7. In apparatus for use in seismic surveying employinga seismometer for converting incoming seismic energy into electrical wave form signals, the combination with means for amplifying and recording such signals, said amplifying'means including thermionic valve amplifiers, of an A. V. C. circuit deriving energy from said signals for applying to a control grid of at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy above a predetermined minimum value so as to maintain the major portion of the record within usable amplitude limits, and means operable by energy flow in said A. V. C..circuit for applying to said circuit a pulsating voltage in substantial phase opposition with the ripple component of saidbias voltage and of comparable magnitude, whereby distortion of the record resulting from'the feeding to the control grid of said ripple component is minimized, said last named means including a thermionic valve having a control grid supplied with voltagefrom said circuit, and means delivering the output of said valve to said circuit.

8. In apparatus for use in seismic surveying employing a seismometer for'converting incoming seismic energy into electrical wave form signaIs the combination with means for amplifying and recording such signals, said amplifying means including thermionic valve amplifiers, of

means for applying to a control grid of .at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy so as to maintain the major portion of the record within usable amplitude limits, said last named means including 1 a triode valve having a control grid fed with a voltage derived from said amplifier, and a condenser charged by the output of said triode valve and supplying control voltage to the grid of said amplifier, and means energized by the output of said triode for returning to said output pulsating energy in phase opposition to said ripple component. 1

9. In apparatus for use in seismic surveying employing a seismometer {or converting incoming seismic energy into electrical wave form signals, the combination with means for amplify ing and recording such signals, said amplifying means including thermionic valve amplifiers, of

means for applying to a control grid of at least one of said amplifiers a bias voltage varying automatically in response to variation in amplitude of incoming energy so as to maintain the major portion of the record within usable amplitude limits, said last named means including a triode valve having a control grid fed with a voltage derived from said amplifier, and a condenser charged by 

